1. Field of the Invention
This invention relates to a stent mounting device and a method of coating a stent using the device.
2. Description of the Background
Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in the patent literature disclosing stents include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
FIG. 1 illustrates a conventional stent 10 formed from a plurality of struts 12. The plurality of struts 12 are radially expandable and interconnected by connecting elements 14 that are disposed between adjacent struts 12, leaving lateral openings or gaps 16 between adjacent struts 12. Struts 12 and connecting elements 14 define a tubular stent body having an outer, tissue-contacting surface and an inner surface.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. Local delivery of a therapeutic substance is a preferred method of treatment because the substance is concentrated at a specific site and thus smaller total levels of medication can be administered in comparison to systemic dosages that often produce adverse or even toxic side effects for the patient.
One method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent. and a therapeutic substance dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
A shortcoming of the above-described method of medicating a stent is the potential for coating defects. While some coating defects can be minimized by adjusting the coating parameters, other defects occur due to the nature of the interface between the stent and the apparatus on which the stent is supported during the coating process. A high degree of surface contact between the stent and the supporting apparatus can provide regions in which the liquid composition can flow, wick, and collect as the composition is applied. As the solvent evaporates, the excess composition hardens to form excess coating at and around the contact points between the stent and the supporting apparatus. Upon the removal of the coated stent from the supporting apparatus, the excess coating may stick to the apparatus, thereby removing some of the coating from the stent and leaving bare areas. Alternatively, the excess coating may stick to the stent, thereby leaving excess coating as clumps or pools on the struts or webbing between the struts.
Thus, it is desirable to minimize the potential for coating defects generated by the interface between the stent and the apparatus supporting the stent during the coating process. Accordingly, the present invention provides for a device for supporting a stent during the coating application process. The invention also provides for a method of coating the stent supported by the device.
The present invention provides an apparatus for supporting a stent during a process of coating the stent. The apparatus includes a member for supporting a stent during the coating process, wherein a section of the member includes a porous surface capable of receiving the coating substance during the coating process. The pores can have a diameter between about 0.2 microns and about 50 microns.
In one embodiment, the member includes a first member for making contact with a first end of the stent and a second member for making contact with a second end of the stent. In such an embodiment, the pores can be located on at least a region of the surface of the first or second members. The first or second member can be made from a metallic material such as 300 Series stainless steel, 400 Series stainless steel, titanium, tantalum, niobium, zirconium, hafnium, and cobalt chromium alloys. The first or second member can also be made from a polymeric material such as, but not limited to, regenerated cellulose, cellulose acetate, polyacetal, polyetheretherketone, polyesters, highly hydrolyzed polyvinyl alcohol, nylon, polyphenylenesulfide, polyethylene, polyethylene terephthalate, polypropylene, and combinations thereof. The first or second member can also be made from ceramics such as, but not limited to, zirconia, silica, glass, sintered calcium phosphates, calcium sulfate, and titanium dioxide. In another embodiment, a layer can be disposed on the surface of the first or second member to absorb coating material that comes into contact with the layer.
In one embodiment, the first and second members have inwardly tapered ends that penetrate at least partially in the first and second ends of the stent and are in contact with the first and second ends of the stent. In another embodiment, the apparatus additionally includes a third member for extending within the stent and for securing the first member to the second member.
The present invention also provides a method of coating a stent. The method includes positioning a stent on a mounting assembly, wherein a section of the mounting assembly includes a porous surface. The method additionally includes applying a coating composition to the stent, wherein at least some of the coating composition that overflows from the stent is received by the pores. The act of applying a coating composition can include spraying the composition onto the stent.
In one embodiment, the method also includes at least partially expanding the stent prior to the act of applying. The method can also include rotating the stent about the longitudinal axis of the stent during the act of applying and/or moving the stent in a linear direction along the longitudinal axis of the stent during the act of applying.
Also provided is a support assembly for a stent. The support assembly includes a member for supporting a stent, wherein the member includes an absorbing layer for at least partially absorbing some of the coating material that comes into contact with the absorbing layer.